Binders useful in electrodeposition painting systems require, in many cases, totally contradictory properties for the various stages of the painting process, which includes the paint manufacture, electrodeposition of the paint, and the stoving process to ensure satisfactory film properties such as good surface quality, high throwing power, and good corrosion resistance. Various other requirements by the consumers, such as a low content of organic solvents or restrictions in the choice of the other paint components, must also be taken into consideration.
Thus, although containing only small amounts of organic solvents, the binders must have a low viscosity to ensure favorable dilution characteristics, including in the bath replenishing process in the electrodeposition plant, and good pigmentability. On the other hand, a high molecular weight of the binders is essential for obtaining good resistance qualities of the crosslinked films, but usually high molecular weight is associated with a high viscosity of the binder. Similarly, contradictory demands exist in the formation of a high electrical resistance in the film during deposition in order to achieve high throwing power and the simultaneous desire for a relatively thick film layer having excellent surface quality. From the literature it is seen that a number of attempts have been made to find a compromise between optimum solubility or dispersibility of the protonized binders and their deposition characteristics, or the properties of the crosslinked paint films by various modifications of bisphenol A or phenol novolak/epoxy resin/amine adducts.
For example, in U.S. Pat. Nos. 4,104,147 or 4,148,772 an epoxy resin based on bisphenol A and epichlorohydrin is reacted, before the reaction with an amine, with a polytetramethyleneglycol, with lengthening of the chain, and in this way a hydrophilic segment is incorporated into the binder molecule. U.S. Pat. Nos. 3,839,252 and 4,035,275 also propose chain lengthening with polypropyleneglycol before reaction with an amine. Another way of introducing polyalkyleneglycol segments consists, according to EP-A2-00 74 634, in lengthening the chain of low molecular bisphenol A/epoxy resins with bisphenol A ethylene oxide adducts.
A major disadvantage of those methods is the difficulty of controlling such chain lengthening reactions since self-condensations of the epoxy resin cannot be totally avoided. Even if the initial products have the theoretical epoxide value, free polyglycols are probably still present which will substantially influence the physical and chemical properties of the binders.
Protonized epoxide/amine adducts based on polyoxyalkyleneglycidylethers, as disclosed for example in U.S. Pat. No. 4,035,275, have proved to be extremely soluble in water. However, these products are difficult to precipitate electrically in the form of usable films and, as might be expected, have serious defects in terms of their film resistance qualities.
Cationically modified epoxy resins which are soluble in water at pH-values of above 7 can be obtained by incorporating quaternary ammonium groups, i.e., by reacting the epoxide groups with tertiary amines in the presence of acids and/or water. Products of this kind, as described for example in U.S. Pat. No. 4,035,275, have in practice proved suitable for use as sole binders only when the specifications are not too stringent. These resins can be used, however, as partial replacement binders.
Austrian Pat. No. 381,115 describes the incorporation of polyoxyalkyl segments in epoxide/amine adducts by reaction of the epoxide/amine adducts with polyoxyalkyleneglycidylethers. Although such segments improve the solubility and dispersibility of the partially neutralized polymers, additional modifiers have to be incorporated in order to ensure that the polymers are sufficiently flexible. Products which are insufficiently plasticized have high glass transition temperatures and defective flow properties of the wet films in the stoving range of from 120.degree. C. to 180.degree. C. This results in serious deficiencies in the "flow," resulting in films of insufficient thickness.
For the purpose of elastification, epoxy resins or epoxy-functional epoxide/amine adducts may be defunctionalized with epoxide-reactive compounds. This can be achieved using monocarboxyl compounds such as monocarboxylic acids of various chain lengths or monoesters of dicarboxylic acids, using longer-chained dicarboxylic acids such as adipic acid and the higher homologues thereof, as well as dimerized fatty acids and the like. To a small extent, polycarboxyl compounds such as maleinized oils or polybutadienes may be used for this purpose. Defunctionalization may also be carried out with polyesters carrying carboxyl groups, with the polyesters optionally being modified with fatty acids. In addition, plastification of the polymer by reacting an epoxide/amine adduct with monoglycidylethers or esters is also possible.
In practice, as the degree of modification increases, all such possible modifications have disadvantages such as a reduction in the wet film resistance and in voltage resistance, and consequently a deterioration in the throwing power; the formation of cleavage products during stoving of the deposited films; reduced resistance qualities of the stoved films, and a deterioration in their adhesion to any subsequent coats.